It is practiced to detect the movement of a person inside a vehicle by using a radar sensor, an ultrasonic sensor, or the like. For example, Tokuhyouhei (Published Japanese translation of PCT application) No. 8-507706 (WO 94/20021) discloses an invention concerning an ultrasonic motion monitor for monitoring the breathing motions of a human being or other mammals. Further, Japanese Unexamined Patent Publication No. 7-204166 and Japanese Patent No. 2827534 disclose that the presence of a human body is detected by detecting the breathing motions of a human by using a piezoelectric element.
There often occur such accidents as a child being left in a vehicle in the intense heat of summer and dying, because of the heat, in the vehicle. To prevent the occurrence of such a tragedy, a system is being studied that detects the presence of a human inside a vehicle, and that issues some kind of alarm when the temperature inside the vehicle becomes excessively high.